Cell-cell interactions play an important role in cell fate decision and pattern formation in the development of multicellular organisms. One of the evolutionarily conserved pathways that plays a central role in local cell interactions is the Notch signal transduction pathway. Initially discovered by developmental biologists working on invertebrates such as Drosophila and C. elegans, the Notch pathway has recently been found to control various developmental processes in mammals. However, the role of Notch signaling in epidermal development, especially its role in epidermal stem cell differentiation, has not been studied. Here I propose to investigate the role of Notch signaling during epidermal development. Specifically I propose to use transgenic mouse technique to hyperactivate or block Notch pathway in the epidermis and to examine their effects on epidermal development. will also develop a sensitive assay to follow the in situ pattern of Notch activation in the developing epidermis. This assay can be generally applied to examine the temporal and spatial pattern of Notch signaling in any developing tissue.